In a medium-sized saucepan, combine the sugar and water. Heat over low heat, stirring until the sugar has dissolved. Once the sugar has dissolved, turn the heat to medium-high and allow the syrup to come to a boil.
In the meantime, add the egg whites to a medium-sized, heatproof bowl and mix (with a mixer fitted with the whisk attachment) until foamy and the whites are almost able to hold soft peaks.
Once the syrup is boiling, clip on a candy (or sugar) thermometer.
Cook until the syrup reaches 116°C/240°F, then take the pan off the heat and slowly drizzle the hot syrup into the bowl with the foamy egg whites, mixing continuously to prevent the eggs from scrambling. Don't pour the syrup onto the whisk, or the syrup may splatter against the sides of the bowl (or into your face!). Instead, aim for a spot close to the whisk.
Once all the syrup has been added, keep mixing until the bottom of the bowl feels cool to the touch and the meringue has cooled down to body temperature.
Use immediately or keep in the fridge (covered) until ready to use. It's a very stable meringue, so it won't start weeping, leaking or collapsing.
Notes
Italian meringue can be made two days in advance and stored in the fridge until needed (covered with plastic wrap).
Recipe by The Tough Cookie at https://thetoughcookie.com/2014/09/11/different-kinds-meringue-make-italian-meringue/